Design of Pd-Based Bimetallic Catalysts for ORR: A DFT Calculation Study

Lihui Ou

doi:10.1155/2015/932616

Abstract

Developing Pd-lean catalysts for oxygen reduction reaction (ORR) is the key for large-scale application of proton exchange membrane fuel cells (PEMFCs). In the present paper, we have proposed a multiple-descriptor strategy for designing efficient and durable ORR Pd-based alloy catalysts. We demonstrated that an ideal Pd-based bimetallic alloy catalyst for ORR should possess simultaneously negative alloy formation energy, negative surface segregation energy of Pd, and a lower oxygen binding ability than pure Pt. By performing detailed DFT calculations on the thermodynamics, surface chemistry and electronic properties of Pd-M alloys, Pd-V, Pd-Fe, Pd-Zn, Pd-Nb, and Pd-Ta, are identified theoretically to have stable Pd segregated surface and improved ORR activity. Factors affecting these properties are analyzed. The alloy formation energy of Pd with transition metals M can be mainly determined by their electron interaction. This may be the origin of the negative alloy formation energy for Pd-M alloys. The surface segregation energy of Pd is primarily determined by the surface energy and the atomic radius of M. The metals M which have smaller atomic radius and higher surface energy would tend to favor the surface segregation of Pd in corresponding Pd-M alloys.

Highlights

Fuel cells have received much attention in recent years as the global effort to reduce our reliance on fossil fuels has increased
The results showed that the active oxygen reduction reaction (ORR) electrocatalysts can be devised without Pt, and their activity can surpass those of pure Pt electrocatalysts [29]
Developing catalysts for ORR with low noble metal contents, high catalytic activity, and high stability is the key for largescale application of proton exchange membrane fuel cells (PEMFCs)

Summary

Introduction

Fuel cells have received much attention in recent years as the global effort to reduce our reliance on fossil fuels has increased. Further enhancement of the ORR catalytic activity and stability of Pd is of considerable interest, and replacing Pt with the less-expensive metal Pd alloy can considerably decrease the operational costs, thereby facilitating the faster and broader application of PEMFCs. In the search for improved alloy catalyst materials, different strategies and schemes have been used, from experimental high-throughput catalyst screen [31] to the study of idealized model systems [8]. For Pd with fully occupied valence d-orbitals, alloying with transition metals such as Co with unoccupied valence d-orbitals significantly reduces the Gibbs free energy both for the first charge-transfer step and for the steps involving the reduction of intermediates These studies have convincingly showed that first-principles catalyst design is a reality for the ORR and corresponding progress has been made in this respect, these arguments for ORR electrocatalysis on Pdbased bimetallic surfaces cannot explain the relatively good activity and stability of Pd-M alloys in acidic medium [37]. Our results were not always quantitatively accurate, the conclusions on trends were reasonably accurate qualitatively because the present experimental phenomenon of the ORR confirmed the computational screening results

Models and Methods

Design Schemes for Alloy Catalysts of ORR

Results and Discussion

ORR Catalytic Activity Analysis of Pd-Based Bimetallic

Conclusions